Re: [RFC PATCH v2 05/47] hugetlb: make hugetlb_vma_lock_alloc return its failure reason

[James Houghton]

On Wed, Nov 16, 2022 at 9:08 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> No objection on the patch itself, but I am just wondering what guarantees
> thread-safety for this function to not leak vm_private_data when two
> threads try to allocate at the same time.
>
> I think it should be the write mmap lock.  I saw that in your latest code
> base there's:
>
>         /*
>          * We must hold the mmap lock for writing so that callers can rely on
>          * hugetlb_hgm_enabled returning a consistent result while holding
>          * the mmap lock for reading.
>          */
>         mmap_assert_write_locked(vma->vm_mm);
>
>         /* HugeTLB HGM requires the VMA lock to synchronize collapsing. */
>         ret = hugetlb_vma_data_alloc(vma);
>         if (ret)
>                 return ret;
>
> So that's covered there.  The rest places are hugetlb_vm_op_open() and
> hugetlb_reserve_pages() and they all seem fine too: hugetlb_vm_op_open() is
> during mmap(), the latter has vma==NULL so allocation will be skipped.
>
> I'm wondering whether it would make sense to move this assert to be inside
> of hugetlb_vma_data_alloc() after the !vma check, or just add the same
> assert too but for different reason.

I think leaving the assert here and adding a new assert inside
hugetlb_vma_data_alloc() makes sense. Thanks Peter.

- James

>
> >
> >       vma_lock = kmalloc(sizeof(*vma_lock), GFP_KERNEL);
> >       if (!vma_lock) {
> > @@ -7026,13 +7026,14 @@ static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
> >                * allocation failure.
> >                */
> >               pr_warn_once("HugeTLB: unable to allocate vma specific lock\n");
> > -             return;
> > +             return -ENOMEM;
> >       }
> >
> >       kref_init(&vma_lock->refs);
> >       init_rwsem(&vma_lock->rw_sema);
> >       vma_lock->vma = vma;
> >       vma->vm_private_data = vma_lock;
> > +     return 0;
> >  }
> >
> >  /*
> > @@ -7160,8 +7161,9 @@ static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
> >  {
> >  }
> >
> > -static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
> > +static int hugetlb_vma_lock_alloc(struct vm_area_struct *vma)
> >  {
> > +     return 0;
> >  }
> >
> >  pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
> > --
> > 2.38.0.135.g90850a2211-goog
> >
> >
>
> --
> Peter Xu
>